Add useExternalSnapshot + useNitroEventReducer + useMessageEventReducer hooks

The three companions promised in docs/ARCHITECTURE.md proposal #1
('Companion to add later') are now in src/hooks/events/:

- useExternalSnapshot wraps useSyncExternalStore for the renderer's
  EventDispatcher.subscribe() + getXxxSnapshot() pairing introduced in
  Nitro_Render_V3 2.1.0.
- useNitroEventReducer and useMessageEventReducer mirror the existing
  *State hooks but collapse multiple event types into a single owned
  state slice. The message variant accepts either a single event type
  or an array; subscription is wired through a single useEffect to keep
  the rules-of-hooks happy.

src/hooks/events/index.ts

@@ -1,6 +1,9 @@
 export * from './useEventDispatcher';
+export * from './useExternalSnapshot';
 export * from './useMessageEvent';
+export * from './useMessageEventReducer';
 export * from './useMessageEventState';
 export * from './useNitroEvent';
+export * from './useNitroEventReducer';
 export * from './useNitroEventState';
 export * from './useUiEvent';